The operation of expansion machines, such as steam turbines, by means of the Organic Rankine Cycle (ORC) method for the generation of electric energy through the use of organic media, e.g. organic media having a low evaporation temperature, which generally have higher evaporation pressures at the same temperatures as compared to water as working medium, is known in the prior art. ORC plants constitute a realization of the Rankine cycle in which electric energy is basically obtained, for instance, by means of adibatic and isobaric changes of condition of a working medium. Mechanical energy is generated by the evaporation, expansion and subsequent condensation of the working medium, and is converted into electric energy. Basically, the working medium is brought to an operating pressure by a feed pump, and energy in the form of heat, which is provided by a combustion or a flow of waste heat, is supplied to the working medium in a heat exchanger. The working medium flows from the evaporator through a pressure pipe to an ORC turbine where it is expanded to a lower pressure. Subsequently, the expanded working medium vapor flows through a condenser in which a heat exchange takes place between the vaporous working medium and a cooling medium. Then, the condensed working medium is fed by a feed pump back to the evaporator in a cycle.
In comparison with water organic media have clearly lower decomposition temperatures, however, i.e. temperatures at which the molecular bonds of the medium break, which results in the destruction of the working medium and in the decomposition into corrosive or poisonous reaction products. Even if the temperature of the live steam is lower than the decomposition temperature of the medium, the latter can be significantly exceeded at locations that are flown through insufficiently, which may occur, above all, in areas of the heat exchanger that are exposed to vapor. Also, a failure of the feed pump entails that the flow through the heat exchanger is interrupted, so that the working medium is directly exposed to the temperature of the heat source employed for the evaporation.
In order to avoid that the working medium is heated to temperature above the decomposition temperature conventional intermediate cycles are used in the ORC plants, in which the heat is transported from the hot medium (flue gas) used for the evaporation through an intermediate cycle to the evaporator. Typically, a thermal oil is used for such an intermediate cycle, whose temperature stability is higher than that of the working medium. The single-phase heat transfer by means of the thermal oil allows a more uniform flow through the heat exchanger in which the working medium is evaporated. This solution shows the following drawbacks, however. Firstly, thermal oils are typically combustible. Therefore, to avoid the oxidation of the thermal oil, the thermal oil cycle has to be provided with a primary nitrogen pressure, which renders the plant technically complicated and expensive. In addition, thermal oils are subject to aging owing to the high thermal load, and have to be replaced at regular intervals. This results in down times of the plant, and in increased costs. Moreover, the electrical performance of the circulation pump transporting the oil results in a considerable reduction of the transferable heat and, thus, of the gained electrical power, in comparison with the direct evaporation of a working medium for which no intermediate cycle is required.
Hence, it is the object of the present invention to provide an improved ORC method which overcomes the above-mentioned disadvantaged and, in particular, can ensure a temperature of the working medium below the decomposition temperature. In the most general sense, it is the object to control the temperature on a heat exchanger such that excess temperatures can be avoided.